1. Field of the Invention
The invention relates to a method and apparatus for producing plastic tubing by extrusion, and specifically to a method and apparatus for controlling the eccentricity in the wall of the tubing during the extrusion process.
2. Description of Related Art
Plastic tubes are commonly produced by an extrusion process in which dry polymeric raw materials are passed to an extruder which employs one or more screw type devices which knead and compress the raw material. Heat is applied in the extruder, and the combination of heat and pressure turn the dry raw material into a molten plastic. At the discharge end of the extruder, the molten plastic is forced through a die, more specifically between an outer die portion and a central die insert.
As the hot plastic tubing exits the die it is passed into a vacuum calibration box which is maintained at reduced pressure and filled with a cooling fluid, typically water. Within the vacuum calibration box is a sizing sleeve or collar, possibly in the form of a series of "wafers," which is smaller in diameter than the tubing exiting the die. Because an axial force is applied to the hot tubing as it exits the die, the tubing is reduced in diameter and thickness before it enters the vacuum calibration box.
The center of the extruded tubing is maintained at atmospheric pressure, while the exterior is subjected to a reduced pressure in the vacuum calibration box. The pressure within the tubing thus tends to expand the tubing against the sizing collar and the result is tubing of a fairly uniform outer diameter.
Average wall thickness is controlled by the ratio of extrusion rate to "haul off" rate. Control of extrusion and haul off has been successfully accomplished using ultrasonic thickness measuring systems to control the ratio very precisely. These measuring systems measure and calculate average wall thickness, and through a feedback loop, adjust either extrusion or haul off rate. The result has been excellent control of average wall thickness. However, these systems have no effect on eccentricity or thickness variations from other causes.
Ideally, the calibration sleeve will be precisely aligned with the die. The axial force induces stress in the extruded tubing, but if the die is precisely aligned with the sleeve, the stresses will have no effect on the form of the extruded tubing.
If the sleeve is not precisely aligned with the die, both an axial stress and a lateral stress will be applied. The resultant vector represents the force line along which polymer chains will tend to align. The lateral force vector applies force to the extruded tubing which tends to move the polymer chains peripherally during the drawing, effectively causing a skewed polymer chain alignment.
Skewing is relatively uniform, increasing the density of polymer chains on the side of the extrudate opposite the direction of misalignment. As a result, the extrudate takes an eccentric form, with a slightly thicker wall on the side opposite and a thinner wall on the side toward which the sleeve has been moved.
However, there are other causes for eccentricity in the tube, particularly differences in pressure between the sides of the die. While this variation can have a number of causes, it is usually the result of debris accumulating on the screen at the entrance to the die. If the accumulation is random across the screen, the pressure profile across the die should be uniform.
Frequently, however, debris will accumulate on one side of the die, thus reducing the pressure on that side and causing higher pressure on the unrestricted side. The high pressure behind the unrestricted side causes the mass flow rate of the extrusion to be significantly greater through that side of the die, and will result in a greater quantity of stretched molecular chains. The longitudinal stress in the tube is much higher on that side, with the unbalance causing a notable bow in the tube. A noticeable eccentricity in the tube can also be seen.
In industrial situations, a minimum wall thickness will generally be specified by the end user. In order to achieve this wall thickness, it will be necessary to provide tubing having a higher average wall thickness which will account for deviations and still allow for the minimum specified wall thickness to be met. When eccentricities occur, this greatly increases the deviation in a portion of the tubing, and requires that the average wall thickness be increased in order to insure that the minimum wall thickness is always met. Thus, it would be highly desirable to minimize eccentricities, which will permit reduction in average wall thickness and a great savings in raw material.
The mounting of a typical prior art extrusion die is shown in FIGS. 1 and 1A. The die 10, is formed of a body 11, an outer die ring 12 and a die insert 14; outer die ring 12 is clamped to the body 11 by retaining bolts 13 and is positioned by four positioning screws 16a through 16d.
In order to prevent leakage of the polymer melt, die ring 12 must be held tightly against body 11 of the die. This is accomplished by providing sufficient axial pre-load on the retaining bolts 13 to prevent internal pressure from forcing the die ring to move away from the body. The pre-load must be high enough to resist the maximum internal pressure which will be applied, and consequently is always present to a greater or lesser degree. Internal pressure variations cause corresponding pre-load conditions to exist.
In addition to the steady state conditions described, there is a significant pressure drop related to the end of the extruder screw flight which is transmitted through the plastic melt to the die ring. As the screw rotates, the pressure drop also rotates. Consequently, at any given point in the melt flow, a drop in pressure can be observed with each revolution of the extruder screw, and the resisting force which must be overcome is also a function of the instantaneous screw position.
If an eccentricity in the wall of a tube is noted, one method for its correction is an adjustment in the outer ring end of the die by adjustment of the positioning screws 16a-16d. In order to move the die ring, the movement of the positioning screws must exert sufficient force to overcome the friction between the die ring and the die body. When an adjustment is made, it is made only in the plane normal to the melt flow, and the screws holding the die ring in the complementary plane are normally left tight to prevent unwanted movement. There are therefore two variable frictional forces which must be overcome in order for the adjustment to be made. The necessary force to be applied to cause movement is a function of the pre-load, the complimentary position screw load and the effective static coefficient of friction between the surfaces which must slide.
Typically, force will be applied by adjustment of a positioning screw until it is great enough to overcome the resisting forces. At the instant that movement begins, the resisting force becomes a function of the dynamic coefficient of friction. Since the dynamic coefficient of friction is always much lower than the static resistance to movement, there will be a sudden and dramatic decrease in the resistance to movement. Movement will therefore continue until the dynamic resistance is less than the force applied by the positioning screws.
Since the resisting forces are variable, the actual movement of the die ring will also be variable. Typically, the best precision which can be attained by a careful operator for a single movement is on the order of 0.010". With a typical draw-down at about 50%, this translates into an eccentricity adjustment of approximately 0.005" in the tube wall. Consequently, eccentricity control by adjustment of the positioning screws is coarse as well as tedious.
In industrial practice, it has been discovered that it is possible to correct eccentricities in the tube by movement of the vacuum calibration box, to obtain a misalignment between the sizing sleeve and the die. While this may result in some ovality to the final tube, the wall thickness can be corrected fairly easily and the ovality can be corrected later on. Movement of the vacuum calibration box has been shown to be a fine adjustment, with a fairly large movement of the box resulting in a fairly small correction to wall thickness. While movement of the vacuum calibration box has been shown to be much easier in actual industrial practice, there have been no known attempts to standardize and automate this practice. Indeed, most of the attempts to standardize and automate corrections have been made with regard to the die itself.